SP4 - Waves

Question 1

What are transverse waves?

Answer 1

Transverse waves are waves in which the particles in the medium vibrate perpendicular (at right angles to) the direction of the wave’s motion.

Question 2

What are longitudinal waves?

Answer 2

Longitudinal waves are waves in which the particles in the medium vibrate parallel to (in the same direction as) the direction of the wave’s travel.

Question 3

What is the frequency of a wave?

Answer 3

The number of waves passing a point each second.

Question 4

What is the period?

Answer 4

The length of time it takes one wave to pass a given point.

Question 5

What is the wavelength?

Answer 5

The distance from a point on one wave to the same point on an adjacent wave.

Question 6

What is the amplitude?

Answer 6

The maximum distance of a point on a wave from rest position (distance from middle to peak or trough).

Question 7

What is the velocity?

Answer 7

The speed at which the wave is travelling.

Question 8

What is the wave speed equation?

Answer 8

v=f/\

Velocity (m/s) = frequency (Hz) x wavelength (m)

Question 9

What is one way to measure the speed of sound?

Answer 9

Stand a known distance away from a wall and make a sound. Time how long it takes for you to hear the echo. Use s=d/t to find the wave speed (note distance will be doubled as the wave has come there and back).

Question 10

Core practical - investigating waves - measuring waves on water

Answer 10

• set up a ripple tank with a straight dipper near one side of the tank. Fasten a ruler to one of the sides so you can see it’s markings above water level.
• vary the voltage of the motor until you get waves with a wavelength of about half as long as the ripple tank
• count how many waves are formed in 10 seconds then divide by 10 to get frequency
• estimate the wavelength of the waves using the ruler
• use v=f/\ to calculate wave speed
• OR mark two points on the tank and measure the distance between them, time how long it takes a wave to pass both points
• use s=d/t to calculate wave speed

Question 11

Core practical - investigating waves - measuring waves in solids

Answer 11

• suspend a metal rod horizontally using clamp stands and rubber bands
• hit one end of the rod with a hammer, hold a smartphone with a frequency app near the rod and note down the peak frequency
• measure the length of the rod and double it to get the wavelength
• use v=f/\ to calculate wave speed

Question 12

What is refraction?

Answer 12

When a wave crosses a boundary at an angle (not 90 degrees) the wave changes speed and appears to bend.

Question 13

When is light refracted towards the normal?

Answer 13

When the light is going from a less optically dense to a more optically dense medium. The wave slows down and refracts towards the normal. The wavelength of the light will increase but the frequency will stay the same as the source is producing the same number of oscillations per second.
For example, air to glass.

Question 14

When is light refracted away from the normal?

Answer 14

When it travels from a more optically dense medium to a less optically dense medium. It speeds up and is refracted away from the normal. It’s wavelength will decrease proportional to the change in speed but the frequency will stay the same as the source is producing the same number of oscillations per second.
For example, glass to air.

Question 15

What is the normal?

Answer 15

A line at 90 degrees to the interface (boundary) between the two mediums.

Question 16

What causes light to be more or less bent (refracted)?

Answer 16

The bend depends on how fast the light travels in the two media and the angle of the light hitting the interface. The greater the difference in speed, the more the light is bent.

Question 17

What 4 things can happen when a wave crosses a boundary?

Answer 17

• reflection (the wave ‘bounces off’)
• refraction (the wave ‘bends’)
• transmission (the wave passes through - often leads to refraction)
• absorption (wave disappears and its energy is dissipated through the material)

Question 18

How can prisms split white light into a spectrum?

Answer 18

White light contains frequencies of all of the the colours of the visible light spectrum. These frequencies change speed different amounts when they go from air to glass or glass to air, causing them to be refracted at varying angles, which causes the white light to be split into the colours of the spectrum.

Question 19

How do humans hear?

Answer 19

• sound waves are funnelled by the outer ear and they enter the ear canal
• they hit the eardrum, causing it to vibrate
• the small bones amplify the vibrations
• vibrations are passed on to the liquid in the cochlea
• the hairs in the cochlea detect the vibrations and create electrical impulses
• these travel along the auditory nerve until they reach the brain, where they are processed.

Question 20

What is the structure of the cochlea like?

Answer 20

The cochlea is a coiled tube containing a liquid. It has a membrane in the middle which is thicker at the base (detects high frequencies) and thinner at the apex (detects low frequencies). There are thousands of hair cells along the membrane, each one connected to a neurone that sends impulses to the brain, which interprets signals from different neurones as different pitches of sound.

Question 21

What is the human hearing range?

Answer 21

20Hz - 20,000Hz

Question 22

What is ultrasound?

Answer 22

Sounds made by waves with higher frequencies than 20,000Hz.

Question 23

What are some uses of ultrasound?

Answer 23

• animal communication and bats echolocation
• sonar (finds depth of sea or fish)
• medical imaging (e.g. pregnancy scanning, cardiology, checking for ligament damage)
• cleaning things

Question 24

How do ultrasound scans work?

Answer 24

A probe emits and receives ultrasound waves. A gel is used to stop the ultrasound being reflected from the skin. Some sound is reflected each time the ultrasound waves pass through a different material (e.g. fat or bone). The ultrasound machine detects the time between sending the pulse out and receiving the echo, the display shows where the echoes came from.

Question 25

What is infrasound?

Answer 25

Sounds produced by waves with frequencies below 20Hz.

Question 26

What are some sources of infrasound?

Answer 26

• animal communications (e.g. elephants)
• supersonic aircraft
• volcanic eruptions
• earthquakes
• explosions

Question 27

What are seismic waves?

Answer 27

Seismic waves are waves that travel through the earth, they cause rock to be moved (earthquakes). They are detected by seismometers.

Question 28

What are the properties of S waves?

Answer 28

• S waves (secondary waves) are transverse seismic waves
• they can only travel through solids
• they are slower than P waves so arrive later

Question 29

What are the properties of P waves?

Answer 29

• P waves (primary waves) are longitudinal seismic waves.
• they can travel through solids and liquids
• they are faster than S waves so arrive first

Question 30

Describe the structure of the Earth.

Answer 30

Crust - relatively thin and rocky (solid)
Mantle - has the properties of a solid but can flow very slowly
Outer core - made from liquid nickel and iron
Inner core - made from solid nickel and iron

Question 31

What is a shadow zone?

Answer 31

The shadow zone of an earthquake is where no seismic waves can be detected.

Question 32

Why are S wave shadow zones much larger than P wave shadow zones?

Answer 32

P waves can travel through solids and liquids, this means that they can be transmitted from one end of the earth to the other, being refracted as they go. However, S waves can only travel through solids and not liquids, so they get absorbed by the liquid outer core and cannot reach the other side of the Earth, making the shadow zone for S waves much larger than that for P waves.